The back light module, also known as the back light source, is an important element of the thin film transistor liquid crystal display (TFT-LCD). The back light module is necessary to provide light beams because the TFT-LCD is a non-self-emitting display panel. The light beams is passed through a first polarizing plate, a first glass substrate, a liquid crystal layer, a color filter, a second glass substrate and a second polarizing plate in order of a TFT-LCD panel. Thus, the light beams are converted into images to user's eyes. So that, a user could see images displayed by the thin film transistor liquid crystal display.
Generally, the back light modules can be classified into the edge-type (edge lighting) back light module and the direct-type (bottom lighting) back light module. The edge-type back light module includes a reflector, light sources (such as cold cathode fluorescent lamps or light emitting diodes), a light guide plate, optical films (such as a diffuser sheet, a prism sheet, and so on). Recently, the light emitting diodes are commonly applied to the edge-type back light module to serve as the light sources. However, if the light emitting diodes are cooperated with laser light sources, the TFT-LCD will have better light emitting efficiency and a larger color gamut.
At present, there are two conventional ways to apply the light emitting diodes and laser diodes simultaneously as the light sources to the edge-type back light module. In one way, the light emitting diodes and the laser diodes are mounted on a same circuit board so as to form a light bar. However, numbers of the light emitting diodes and the laser diodes mounted on the circuit board are limited by the mounting space of the circuit board, thus the light sources can't provide enough brightness for the edge-type back light module. In another way, the edge-type back light module includes two light guide plates, wherein the two light guide plates overlap with each other. The light emitting diodes are disposed beside one of the light guide plates, and the laser diodes are disposed beside the other one of the light guide plates, so that the numbers of the light emitting diodes and the laser diodes can be increased. However, in this way, the manufacturing cost and the thickness of the back light module are increased. In addition, if the two light guide plates have different luminance distribution, a chromaticity is varied in different regions. U.S. Pat. No. 7,303,322 discloses a “multiple lightguide backlight” which also has above problems, that is, the manufacturing cost is high, and a chromaticity is varied in different regions due to the two light guide plates having different luminance distribution. Therefore, to resolve or improve above problems has become a main topic in this field of technology.